1. Field of the Invention
The present invention relates to a plastic substrate and more particularly relates to a transparent plastic substrate for use in optical instruments and a liquid crystal display device including such a substrate.
2. Description of the Related Art
A flat panel display such as an LCD is expected to have even a lighter weight, a further reduced thickness and a higher degree of shock resistance.
To meet these demands, techniques of using a plastic substrate instead of the conventional glass substrate have been proposed. The plastic substrate may be made of either a thermosetting resin such as a polyimide resin or an epoxy resin or a thermoplastic resin such as polycarbonate.
Generally speaking, the resin is inferior to the glass in thermal resistance and dimensional stability, among other things. For example, a plastic substrate usually has a resistant temperature of approximately 250° C. or less, although the temperature may vary somewhat with its resin material. And the resistant temperature is lower than a process temperature with a glass substrate by 100° C. or more. In various process steps of forming circuit components, including electrodes, interconnects and semiconductor devices, on a plastic substrate (e.g., in a film deposition process step, in particular), the process temperature cannot be decreased beyond a certain limit. That is why the development of a plastic substrate with high thermal resistance, which is much closer to that of the conventional glass substrate, is awaited. Also, depending on its intended application, the plastic substrate may need to have almost as high colorlessness and transparency as the glass substrate.
Most of the plastic substrates on today's market cannot satisfy all of these requirements at the same time. For example, a polyimide resin normally has excellent thermal resistance but its color is a problem. Meanwhile, resins with good colorlessness and transparency often exhibit poor thermal resistance.
As a measure for increasing the thermal resistance of a plastic substrate, somebody proposed that a protective coating be provided on the surface of a substrate made of a resin (which will be referred to herein as a “resin substrate”).
Another person proposed a method of making a plastic substrate of a composite material, in which a resin and a filler are mixed together, in order to increase the thermal resistance and dimensional stability thereof. A substrate made of a composite material will be referred to herein as a “composite substrate”. For example, Japanese Patent Application Laid-Open Publication No. 11-2812 discloses a reflective conductive substrate including a composite substrate, which is obtained by impregnating a piece of glass fiber cloth with a resin.
On the other hand, Japanese Patent Application Laid-Open Publication No. 2001-133761 discloses a plastic substrate including a composite substrate in which a plurality of linear or striped fibers are arranged in a resin so as not to contact with each other. According to Japanese Patent Application Laid-Open Publication No. 2001-133761, if a composite substrate with a fiber cloth (i.e., woven fabric) embedded as disclosed in Japanese Patent Application Laid-Open Publication No. 11-2181 mentioned above is used, then very small unevenness is created on the surface of the substrate by either the textured or overlapped portions of the fiber cloth, thus causing deterioration in resultant display quality. However, according to this publication, a composite substrate with a flat surface can be obtained by adopting its proposed arrangement.
The present inventors analyzed the optical properties of such a composite substrate, in which fibers were embedded in a resin matrix, from various angles. As a result, we discovered that when an LCD was fabricated with such a conventional composite substrate, leakage of light occurred due to a retardation associated with the direction in which the fibers were arranged in the composite substrate (i.e., the direction defined by the major axis of the fibers), thus interfering with high-quality display.
That is to say, the present inventors discovered that the conventional fiber-embedded composite substrate had a retardation associated with the fiber arrangement direction. And we discovered that the conventional composite substrate would cause deterioration in display quality when used to make an LCD because its retardation had not been adjusted to a predetermined value. In other words, even if the unevenness caused by the textured or overlapped portions of the fiber cloth could be eliminated as disclosed in Japanese Patent Application Laid-Open Publication No. 2001-133761, the display quality would still decrease unless the retardation is controlled to its predetermined value. Japanese Patent Application Laid-Open Publication No. 2001-133761 does mention the distribution of refractive indices but is silent about retardations (phase differences) and its distribution. Thus, they don't seem to have known that the fiber-embedded composite substrate had a retardation associated the fiber arrangement direction.
In the foregoing description, problems to arise when a plastic substrate is used in an LCD have been discussed. However, those problems caused by the plastic substrate's uncontrolled retardation are encountered no just in LCDs but also in other types of optical instruments, e.g., optical instruments utilizing polarization, in particular.